Short-range mobile radars have several applications. Mention may notably be made of:                radars for detecting the arrival of missiles, these radars being fitted to aircraft or any other platform;        radars fitted to so-called “Sense and Avoid” systems;        radars fitted to automobiles, for speed regulation or anticollision functions for example;        radars for detecting nearby objects.        
For cost reasons, such radars frequently use a waveform having very few distance gates, and even in certain cases a single distance gate. These radars may be of the pulsed wave or continuous wave type.
Conventionally, a pulsed radar deduces the distance to an object from the delay between the instant of emission of the wave and the reception of its echo. In practice, a certain number of distance gates whose duration is close to the inverse of the band of the signal emitted is used. The distance is deduced from the position of the distance gate corresponding to the maximum signal level received and it can be refined on the basis of the relative measurement of the level of the signal received in the adjacent gates.
Another known scheme for measuring distance is to emit a sequence of at least two emission frequencies that are slightly shifted, generally of the order of a kHz to a hundred kHz. The emitted wave is called FSK, the acronym standing for “Frequency Shift Keying”. In the case where two frequencies F1 and F2, spaced δF apart, are emitted alternately, the differential phase shift between the two returns corresponding respectively to the pulses at the frequency F1 and to the pulses at the frequency F2 is given by the following relation:
                    δφ        =                              4            ⁢            π            ⁢                                          R                ⁢                                                                  ⁢                δ                ⁢                                                                  ⁢                F                            c                                +                      2            ⁢                                                  ⁢            π            ⁢                                                  ⁢                          F              D                        ⁢            Δ            ⁢                                                  ⁢            T                                              (        1        )            Where R represents the distance to the target, c the speed of light, FD the Doppler effect due to the relative mobility of the target and ΔT the time interval between the successive emissions of the sequences at F1 and F2.
The distance R is therefore obtained by measuring the phase difference Δφ after having eliminated the residual bias due to the mean Doppler effect FD by virtue of Doppler analysis and by assuming that there is no Doppler ambiguity. The maximum distance measured is attained when Δφ=2π:
                    R        <                  c                      2            ⁢                                                  ⁢            δ            ⁢                                                  ⁢            F                                              (        2        )            
These solutions exhibit several drawbacks. In particular if the frequency gap δF is large, the distance measurement is accurate, for a given signal-to-noise ratio, but the measurement rapidly runs the risk of being ambiguous on a remote target with a radar cross section large enough to produce a detection. Conversely if δF is small, the risk of ambiguous distance measurement is low but this measurement is rather inaccurate.